The present invention relates to single screw mechanisms and more particularly to single screw mechanisms utilizing multiple tooth engagement.
In order to form single screw mechanisms it is known to make use of combinations comprising a mainrotor having a conical surface and projecting threads having a generally helicoidal shape, e.g. U.S. Patent to Zimmern Re No. 30,400. The crests of the threads are intended to cooperate with a casing, thereby forming compression chambers, and the mainrotor is adapted to cooperate with one or a number of gaterotors, the teeth of which are in meshing relation with the threads formed on the mainrotor.
The space formed between two adjacent threads of the mainrotor form a compression chamber which is sealed off at one end by a gaterotor tooth and at the other end by a casing closed end.
When fluid is sucked into the compression chamber, the rotation of the mainrotor permits a progressive reduction in the volume of the compression chamber, compressing the fluid until the compression chamber is put into communication with a casing outlet.
Current practice in the design and manufacture of single screw mechanisms to utilize only a single gaterotor tooth to seal off the mainrotor compression chamber at the intake side. Because of this practice the single gaterotor tooth must fit very closely in the mainrotor compression chamber to minimize internal leakage and to withstand the high differential pressure forces applied to the single gaterotor tooth. These factors have made necessary the use of thick nonmetallic gaterotor teeth, capable of plastically conforming to thread profiles, backed by metallic supports on the low pressure side, to provide adequate stiffness. An example of supported gaterotor teeth as are well known in the art is shown in German Pat. No. 28 33 292 to Zimmern issued Feb. 8, 1978 as shown in FIG. 2 of said German patent.
The casing window area required for the meshing of these thick gaterotor and support combinations with the mainrotor has limited the angle that mainrotor threads may wrap around the main rotor, i.e. wrap angle, to less than 360 divided by the number of gaterotors and limited the mainrotor to gaterotor gear ratio to a maximum of 6 to 17. Such low wrap angles create stronger discharge line pulsations, because only one gaterotor tooth seals each compression chamber, and low gear ratios create poor gaterotor tooth sealing shapes and high rubbing forces.